Letter

Electrical impedance tomography has become a tool for monitoring regional ventilation.
Interest is growing to derive additional information on pulmonary perfusion and ventilation/perfusion
distribution. Since signals recorded by electrical impedance tomography also contain
cardiac-related impedance changes, attempts are made to evaluate them in view of perfusion.
Recently in Critical Care a corresponding study applying an advanced filtering technique for separating cardiac
signals from the dominant ventilation component was published [1]. The quality of the concept is demonstrated by comparing the results with solitary
cardiac signals during breath-hold.

Similar to other publications dealing with this approach, the correct term cardiac-related
impedance changes is substituted by perfusion. Although major aspects of limitations
and missing validation are addressed in the discussion, the key message presented
is that 'it is possible to distinguish between lung ventilation and perfusion using
electrical impedance tomography'.

Vascular volume pulses which are the reasons for cardiac-related impedance changes
in the lung area obviously do not unequivocally represent perfusion. Even peripherally
occluded arteries will pulsate without flow. These pulsations are dependent on blood
pressure and vascular as well as surrounding tissue compliance.

A reasonable approach to assess perfusion by electrical impedance tomography is the
impedance indicator dilution technique, which is clearly favoured by recent publications
[2-4]. In contrast to the questionable pulsation approach, a homogeneous distribution of
perfusion and ventilation/perfusion ratios was obtained [5].

In summary, the interpretation of cardiac-related impedance changes in view of perfusion
will not be permissible until validation by established reference techniques including
relevant pathophysiological conditions.